42(1) Subsection (2) is about the amount of credit allowed under section 18(2) against corporation tax to which a company is liable in respect of any income or chargeable gain.
42(2) The credit must not exceed–
R × IG
R is the rate of corporation tax payable by the company, before any credit under this Part, on the company's income or chargeable gains for the accounting period in which the income arises or the gain accrues, and
IG is the amount of the income or gain (but see subsection (3)).